Or arch. On the west side of the Dolores River roughly a mile and a half downstream from its confluence with the San Miguel. The feature is marked on the USGS Red Canyon 7.5' topo map, but is not named. Telephoto view across the river; map position is that of the arch. Colorado, USA.
Update From an online source (https://www.atlasobscura.com/places/horse-collar-arch) I've learned that this is called Horse Collar Arch.